12 mats in Edoma (江戸間, Kantō) — 18.59 m²

Same count, other standards

Six mats in the widest Kyōma make 10.94 m²; in the narrowest Danchima, 8.67 m². That is 26 %, and the ratio does not change with the count — the areas scale together, so it cancels. Filling a Kyōma six-mat room with Danchima mats would take 7.6 of them.

All five standards are exactly 2:1

The long side is exactly twice the short one, because two half mats must make a whole for the floor to tile. So a mat’s area is twice the square of its short side, and that holds across every standard — only the short side changes: 955 mm for Kyōma, 910 for Chūkyōma, 850 for Danchima.

A tsubo is two mats

One tsubo is 400/121 square metres, or 3.3058 m². Two Chūkyōma mats come to 3.312 m² — within 0.2 %, which is the trace of the unit’s origin in tatami. That is why six mats are loosely called three tsubo and eight mats four; how far the shorthand drifts depends on which standard was laid.

Nearby mat counts

Same standard, other counts

When renting, the mat count is not enough

Two listings that both say “six mats” can differ by more than two square metres. Danchima in particular was cut down to fit apartment blocks and is smaller even than Tokyo’s Edoma. Japanese listings usually print the square metres too — that is the number to read.

How to read this

• Mat area = short side × long side. In all five standards the long side is double the short.
• Floor area = mat area × number of mats.
• At the same count, standards differ by as much as 26 %.
• One tsubo is 3.3058 m², which two Chūkyōma mats very nearly make.